The present invention relates to a reflective and/or refractive secondary lens system for focusing sunlight onto semiconductor elements, the secondary lens system being characterised according to the invention by a projection which is disposed around the basic body forming the secondary lens system. Furthermore, the present invention relates to a semiconductor assembly which includes the secondary lens system according to the invention, and also to a method for the production of this semiconductor assembly. In particular, this semiconductor assembly represents a concentrating solar cell module.
In concentrator photovoltaics, light is concentrated onto solar cells by means of an optical system. For this purpose, for example a lens or a Fresnel collector which bundle the incident light onto the solar cell is used. A plurality of solar cells is assembled with the associated optical system, e.g. a lens array, and also elements for cooling and for electrical wiring to form modules. These modules are mounted on so-called trackers on which they are made to track the course of the sun.
In concentrator photovoltaics, it is of great interest that as much as possible of the radiated light impinges on the solar cell. This is influenced, on the one hand, by the imaging quality of the optical system, on the other hand, by the accuracy with which the optical system is orientated towards the cell and also the module in total towards the sun.
A further important aspect in concentrator photovoltaics is the so-called concentration factor. This indicates the ratio of the light entrance surface of the lens system to the active surface of the solar cell. In order to use as little as possible of the relatively expensive solar cell surface, the concentration factor is chosen to be as large as possible. Precisely in the case of highly concentrating systems, the use of a two-stage lens system is possible, both elements of which are then termed primary lens system (first optical element in the beam path, e.g. the lens or the Fresnel collector) or secondary lens system (second element). A two-stage concept has the advantage that the beam deflection for each individual element can be smaller. In addition, the configuration clearance in the lens design is significantly increased, e.g. for reducing the chromatic aberration or for homogenising the incident radiation.
The secondary lens system has to date generally been configured as a refractive element in which the light beams are directed onto the solar cell by internal total reflection. Elements made of glass are known here in the form of a truncated pyramid (U.S. Pat. No. 5,505,789) or more complex forms which are based mainly on total reflection and are produced in the injection moulding process (e.g. ES 2232299; V. Diaz, J. Alarez, J. Alonso et al., “Assembly of Concentrator Modules based on Silicon Solar Cells at 300× of Concentrated Sunlight”, Proc. of 19th European Photovoltaic Solar Energy Conference, 2004). In order to achieve as little reflection as possible on the exit surface, caused by great differences in the refractive index, this element is normally mounted directly on the solar cell via an adhesive and optically transparent material, e.g. silicone, and is provided possibly with a reflection-reducing coating on the entrance aperture.
At the same time, also simple secondary lens systems which are based on reflection on reflective surfaces are used. In the previously known applications, trapezoidal metallic bodies or round funnels are used (see e.g. EP 0 657 948 A2; WO 91/18419; L. M. Fraas, “Line-Focus Photovoltaic Module Using Stacked Tandem-Cells”, 1994). In order to increase the refection of these components, the metal sheets are frequently provided with highly reflective layers before reshaping. Such constructions are known for example from U.S. Pat. No. 5,167,724 or U.S. Pat. No. 5,505,789 and reproduced by illustration in FIG. 1. The secondary lens system 50 is fitted thereby directly on the solar cell 2. Sunlight is thus prefocused onto the secondary lens system by the Fresnel collector 15.
Furthermore, components are known according to the state of the art (WO 2004/077558 A1; DE 195 36 454 A1; DE 199 47 044 B4), which are designed to be used in conjunction with semiconductor elements in which the radiation exits or is received entirely or partially on the lateral surfaces. This lateral radiation is characteristic of LED semiconductor chips. However, if semiconductor elements which emit or receive almost exclusively more than 95% of the radiation towards the upper side of the semiconductor chip, such as e.g. solar cells, are used, then part of the radiation is lost with the reflector concept according to these patents since the reflectors are designed such that the semiconductor chip is inserted from the top into the reflector recess and hence both lateral walls and upper contacting surface are in the beam course of the reflector. In the case of this construction, the reflecting regions enclose the entire semiconductor chip.
An element in which the transmitter and/or receiver is surrounded by a metallic reflector is known for example from DE 199 47 044 B4. The integral shaping of the reflector walls from the conductor strip material, known from this publication, is also based on the principle that the chip can be inserted into the reflector, i.e. the chip is smaller than the smallest reflector diameter/reflector cross-section.
A tub-shaped configuration of a reflector into which the semiconductor chip is inserted is likewise known from DE 195 36 454 A1.
An element in which a reflector is produced by metallisation of a housing body is known from WO 2004/077558 A1. Here also, the semiconductor chip is applied on a first region of the metallisation. If only a part of the chip surface is intended to be situated in the exit aperture of the reflector, this construction is not suitable.
This state of the art, with respect to the refractive secondary lens systems, has the following disadvantages:                Due to absorption in the material of the secondary lens system, part of the light is absorbed and therefore is no longer available for conversion in the solar cell.        Due to the absorption of the light in the material, the material heats too greatly so that, in particular in the case of highly concentrating systems, the result can be destruction of the secondary lens system.        At the entrance surface of the refractive lens system, the result is reflections due to the high refraction difference relative to the ambient air. These can in fact be reduced by antireflection coatings but these increase the manufacturing costs and can only reduce reflections but not prevent them.        The principle of total reflection (total internal reflection, TIR) places very high requirements on the surface quality of the components. This acts as a strong cost driver in production since the manufacturing methods favoured for large scale production, such as reshaping or injection moulding, and the surface qualities which can be achieved therewith frequently are inadequate. By means of grinding, the surface qualities can be achieved, however this is a relatively expensive process in high-volume manufacturing and is not compatible with the permissible costs in concentrator photovoltaics.        In order to avoid reflections, the space between cell and secondary lens system is filled, as described, with an optical medium (see e.g. ES 2232299, U.S. Pat. No. 5,505,789). In order to minimise air inclusions, this medium is normally applied in viscous state and hardened after assembly of the secondary lens system. Due to capillary effects or wetting effects, the result is however frequently wetting of the outside wall of the secondary lens system with the liquid medium, as a result of which the efficiency of the TIR is reduced. Because of the surface tension of the optical medium, the result in the region of the edges of the secondary lens system is formation of characteristic gaps which likewise lead to uncoupling of light and hence to reduction in efficiency.        Since the secondary lens system must, as a condition of the principle, cover the entire solar cell surface, all beams which are intended to be directed onto the solar cell must also previously pass through the secondary lens system. However, in the case of beams which would impinge on the cell even without this lens system, this leads to unnecessary losses. Precisely in the case of very good primary lens systems, a large part of the beams impinge on the cell even without a secondary lens system. In this case, a secondary lens system, which operates in addition to the primary lens system and detects merely the part of the beams which would not impinge on the cell without further intervention, is therefore optimal.        
With respect to previous designs of reflective secondary lens systems, the following disadvantages should be mentioned:                The known secondary lens systems are difficult to mount since there are no elements which facilitate automatic engagement or simplify the mounting on the cell.        The mounting methods known from WO 91/18419, ES 2232299 or WO 2006/130520 A2 are based on using numerous additional mounting aids, such as e.g. screws, frames or clamping saddles. This drives up the material and process costs and increases the number of components susceptible to faults and hence the probability of the entire system failing.        With respect to the reflectors for semiconductor chips from (WO 2004/077558 A1; DE 195 36 454 A1; DE 199 47 044 B4), the main disadvantage is that the components described there are designed such that the semiconductor chip is placed in a reflecting shaped portion. As a result, only a relatively undefined beam course can however be produced, also the lateral walls or the surface metallisation, according to the incident/emergent beam angle, are situated in the beam course.        Since the reflection layers on the metal sheets tear in the reshaping process when the reshaping is too severe, the possibility of producing special geometries is greatly restricted.        Due to absorption in the material, the secondary lens systems can heat up greatly. To date, the use of special elements to increase the heat dissipation has only been documented in one application (WO 91/18419) in which the heat dissipation is however produced via an additional component which is complex to produce.        The reflective layers (e.g. silver-based layer systems) have high susceptibility to corrosion. In order to prevent this, the reflecting metal sheets are provided with a passivation layer. However, since the components are cut from ready coated metal sheet strips, the cut edges have material transitions which are open and on which the reflective layers are not passivated. These cut edges form the seed cells for corrosion during operation.        Further disadvantages can be found in the corresponding patent quotations in the documentation for the state of the art.        